Make the GUI viewer/compressor application much more usable

Implement several improvements to make the GUI application
(texview) more practical:

Add an estimated-time-of-arrival indicator and display the texture
filename in the title bar.

The GUI update frequency during compression has been adjusted lower
and is now configurable in compression settings. This should help
eliminate any increase of compression time due to frequent GUI
updates, particularly for large textures.

Update Makefile to include install option.

Show the current source filename in the window title, and try to guess
the output filename for the saving dialog.

Add menu option to clear the result image set.

Bump version to v0.7.

The command-line utility (texgenpack) is not affected by this commit.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>