Allow certain windows to float by default in tiling mode

Certain types of windows do not lend themselves well to tiling, and usually work better when allowed to float even in an otherwise tiling setup.

Examples:

• Gnome settings panel (forces fixed width/height on its windows; mainly used in a transient way where the tiled workspace is not of interest as long as the settings window is open anyway)
• Synapse launcher and similar programs. The small launcher/search window (launched by keyboard shortcut) should not be tiled, as this disturbs the tiled layout very much and the user typically expects the (transient) window to be found overlaid in the center of the screen.

In vanilla awesome, this works as expected by default, but not in material-awesome.