Selectable: add support for specifying text alignment on selectables (#2347)

Adds a style variable to Selectable that allows clients to specify the
text alignment within Selectables, adds a section in the demo to
demonstrate selectable text alignment, and a pair of sliders in the
style editor to change selectable alignment on the fly.

In terms of implementation, this one is extremely simple: Selectable was
already calling an API that supports text alignment, but had hard-coded
it to top-left. This changes that to just pass the style variable
straight through to RenderTextClipped. Backwards-compatibility is
preserved by defaulting the text_align parameter to (0, 0), i.e.,
top-left.

This also fixes a bug with selectable text rendering that caused
right-aligned text in a selectable to be clipped incorrectly, because
the wrong clipping rectangle was being used.

imgui.h

@@ -1082,6 +1082,7 @@ enum ImGuiStyleVar_
     ImGuiStyleVar_GrabRounding,        // float     GrabRounding
     ImGuiStyleVar_TabRounding,         // float     TabRounding
     ImGuiStyleVar_ButtonTextAlign,     // ImVec2    ButtonTextAlign
+    ImGuiStyleVar_SelectableTextAlign, // ImVec2    SelectableTextAlign
     ImGuiStyleVar_COUNT
 
     // Obsolete names (will be removed)
@@ -1263,6 +1264,7 @@ struct ImGuiStyle
     bool        AntiAliasedLines;           // Enable anti-aliasing on lines/borders. Disable if you are really tight on CPU/GPU.
     bool        AntiAliasedFill;            // Enable anti-aliasing on filled shapes (rounded rectangles, circles, etc.)
     float       CurveTessellationTol;       // Tessellation tolerance when using PathBezierCurveTo() without a specific number of segments. Decrease for highly tessellated curves (higher quality, more polygons), increase to reduce quality.
+    ImVec2      SelectableTextAlign;        // Alignment of selectable text when selectable is larger than text. Defaults to (0,0) for top-left alignment.
     ImVec4      Colors[ImGuiCol_COUNT];
 
     IMGUI_API ImGuiStyle();