feat: introduce binding generator

A squash of hundreds of commits with a binding API generator.

The API generator works with a hybrid model:
1. We define methods and fields in the code.
2. Mark them with API annotations.
3. For specific use cases, we use the old approach with JNI generation comments.

There is also an API to generate an AST tree, which is saved to JSON.
This JSON can be used for full API generation by declaring only field/method names.
Currently, it is used to generate constants.

By eyeballing, 95% of the API has been converted to the new approach.
This is sufficient for the current moment. Only edge cases with specific API calls are left.

fix: ambiguous call for ImGui::GetColorU32

build: disable checkstyle for imgui-binding module

Since the module is heavily generated, checkstyle for it doesn't make sense.

build: make buildSrc to use JDK 11

build: make generateApi optional when compiling project

Initially generated module was not included to the project VCS.
Since it's now in the separate src folder, we don't need to call generator every time.
Only when a new generation result is required.

imgui-binding/src/generated/java/imgui/flag/ImGuiPopupFlags.java

@@ -0,0 +1,84 @@
+package imgui.flag;
+
+
+/**
+ * Flags for OpenPopup*(), BeginPopupContext*(), IsPopupOpen() functions.
+ * - To be backward compatible with older API which took an 'int mouse_button = 1' argument, we need to treat
+ *   small flags values as a mouse button index, so we encode the mouse button in the first few bits of the flags.
+ *   It is therefore guaranteed to be legal to pass a mouse button index in ImGuiPopupFlags.
+ * - For the same reason, we exceptionally default the ImGuiPopupFlags argument of BeginPopupContextXXX functions to 1 instead of 0.
+ * - Multiple buttons currently cannot be combined/or-ed in those functions (we could allow it later).
+ */
+public final class ImGuiPopupFlags {
+    private ImGuiPopupFlags() {
+    }
+
+    /**
+     * Definition: {@code 0}
+     */
+    public static final int None = 0;
+
+    /**
+     * For BeginPopupContext*(): open on Left Mouse release. Guaranteed to always be == 0 (same as ImGuiMouseButton_Left)
+     *
+     * <p>Definition: {@code 0}
+     */
+    public static final int MouseButtonLeft = 0;
+
+    /**
+     * For BeginPopupContext*(): open on Right Mouse release. Guaranteed to always be == 1 (same as ImGuiMouseButton_Right)
+     *
+     * <p>Definition: {@code 1}
+     */
+    public static final int MouseButtonRight = 1;
+
+    /**
+     * For BeginPopupContext*(): open on Middle Mouse release. Guaranteed to always be == 2 (same as ImGuiMouseButton_Middle)
+     *
+     * <p>Definition: {@code 2}
+     */
+    public static final int MouseButtonMiddle = 2;
+
+    /**
+     * Definition: {@code 0x1F}
+     */
+    public static final int MouseButtonMask_ = 31;
+
+    /**
+     * Definition: {@code 1}
+     */
+    public static final int MouseButtonDefault_ = 1;
+
+    /**
+     * For OpenPopup*(), BeginPopupContext*(): don't open if there's already a popup at the same level of the popup stack
+     *
+     * <p>Definition: {@code 1 << 5}
+     */
+    public static final int NoOpenOverExistingPopup = 32;
+
+    /**
+     * For BeginPopupContextWindow(): don't return true when hovering items, only when hovering empty space
+     *
+     * <p>Definition: {@code 1 << 6}
+     */
+    public static final int NoOpenOverItems = 64;
+
+    /**
+     * For IsPopupOpen(): ignore the ImGuiID parameter and test for any popup.
+     *
+     * <p>Definition: {@code 1 << 7}
+     */
+    public static final int AnyPopupId = 128;
+
+    /**
+     * For IsPopupOpen(): search/test at any level of the popup stack (default test in the current level)
+     *
+     * <p>Definition: {@code 1 << 8}
+     */
+    public static final int AnyPopupLevel = 256;
+
+    /**
+     * Definition: {@code ImGuiPopupFlags_AnyPopupId | ImGuiPopupFlags_AnyPopupLevel}
+     */
+    public static final int AnyPopup = 384;
+}