Created CONTRIBUTING.md with some basic information

There's stuff on issue reporting and also a new style guide. These will
probably both undergo a lot of revisions and expansions; C++ doesn't
even have a style guide yet.
Fix exception with colour popups

Work around an assertion with the colour popups. For some reason, the
button release handler was getting invoked twice. Fixes #123.
Removed a number of glob imports

Glob imports meaning `from x import *`. I don't think it's called that
in python.

Glob imports are near-universally bad, as not only do they make
statically detecting undefined variables impossible, they're also just a
bad development practice to begin with.

Quite a few remain in gui/colors/, but the imports in there are a
complete and utter mess so I'm planning to refactor it around.
Fix floating window title bars in Ubuntu

Fix broken decoration flags. Ubuntu 14.04 wasn't showing window titlebars,
for floating windows making them impossible to move.

Remove some dead code.

Update file copyright.
Remove keep-above forcing in fullscreen mode

Window managers are a pain.

Forcing keep-above for the main window in fullscreen mode breaks
fullscreening entirely in Awesome WM. Therefore, rip out this hack:
workarounds that completely break functionality that would otherwise work
will be dropped.

Xfwm4 will be annoying now if there are subwindows visible in fullscreen
mode, so it's best to use the docks on this wm and unfullscreen when using
the preferences window. I use xfwm4 myself. Sorry for the inconvenience.

Unity doesn't seem too badly affected either way these days.

Fixes #89.
Merge branch 'misc-code-cleanup'
Merge pull request #124 from iirelu/contributing-guide

Created CONTRIBUTING.md with some basic information
Only auto-enable the frame on the first activation

Fixes #79.

The frame tool now keeps track of whether or not it's being activated
for the first time. Previously, once it was switched back to after a
panning/rotating/etc operation, the frame was automatically enabled
regardless of whether it was before, which was very frustrating.

However, if the frame is disabled and the user manually switches to the
frame tool, it'll still be re-enabled. I'm not sure why this happens,
but it is actually a desirable effect, as to the user functionality is
near-identical but with one less bug.
Added a button in the debug menu to cause a crash

It just calls `raise Exception("This is a crash caused by the user.")`
Added more information to crash logs

It now has details on the current Mypaint version being used and what
operating system it's running on. The system information is provided by
the platform library.