app: only enable the resolution chain button if xres and yres are the same
app: enable image new's resolution chain button only if xres == yres
app: enable scale image's resolution chain button only if xres == yres

(the edit_resolution feature of GimpSizeBox is currently used
only by the scale image dialog)
Bug 724444 - Printing ignores dpi ratio / image proportions, always prints 1:1

Enable the resolution chain button only if xres == yres.
app: cancel the active tool before going brightness/contrast->levels->curves

because changing tools now automatically commits the old tool.
app: implement the gimp_overlay_dialog_set_response_sensitive() stub
app: FG select: rename "Reset" to "Cancel" because that's what it does

Also call update_gui() at the right places so we don't leave the GUI
in an inconsistent state.
app: FG select: remove trailing space from the "status_mode" message

such formatting should not be done intranslatable strings, instead
add the space between the two messages when putting them together
using an untranslatable format string.
app: FG select: reorder functions to match all other tools
app: FG select: refactor to use the standard tool cancel/commit logic

Simplifies things and prevents losing a complex selection on tool change.
libgimpwidgets: add a string to GIMP_STOCK_TOOL_FOREGROUND_SELECT
app: FG select: use the FG select stock icon/text instead "Apply"

and GTK_RESPONSE_APPLY instead of a private RESPONSE_APPLY.
app: FG select: turn the "Preview" button into an actual toggle button
app: make the transform tool's undo functions static
Bug 312780 - Add undo to foreground selection tool

Implement undoing strokes by keeping copies of the changed parts
of the trimap around in internal undo/redo stacks.
app: FG select: default to drawing foreground strokes
app: FG select: refresh the preview when matting options change
app: FG select: remove some redundant "display" parameters
app: use the right aspect and angle actions for GimpBrushTool
app: rename the value-1...value-4 actions to opacity, size, aspect, angle