xfwm4: bigger buttons, top left/right px clickable

Allow the top left and right pixels of maximized windows to be clickable
if there's a window button there, for greater Fitts's Law compliance. The
exception is the window menu button, which needs a minor hack to be sized
correctly.

Somewhat darker inactive button icons, to match window title text.

Add an SVG-based build system to allow colours to be changed easily.
xfwm4: increase width of bottom edge

Increase the size of the bottom resize edge, and make the left-bottom and
right-bottom corners larger, and "L"-shaped, for easier gripping.

Rationale: Xfwm doesn't support magic window edges, so they need to be
larger in order to be clickable. 2px is too narrow for this.
xfwm4: thicker window borders

The style looks more balanced and true to upstream with the 4px borders all
around.
Merge pull request #96 from achadwick/xfce4-usability

Xfce4 usability