Redesign Image -> Mode menu as Image -> Transparency

Since I have no plans to offer modes other than 24/32bpp, it was a poor
original decision to stick transparency under "Mode".  With this commit,
"Mode" has effectively been replaced with "Transparency".  I believe
this is much clearer to the user, and it will also allow me to add
additional alpha channel options in the future - in a menu where such
options actually make sense!
Redesign Image menu using control indices

I've put off redesigning this menu for a long time, as there are a lot
of enable/disable nuances contingent on the current image.  Good to
finally have it done!
Disable copy-to-clipboard metadata testing