Respect minimum size hints for floating windows. (#2508)

This commit introduces proper support for the minimum size on floating
windows by ensuring that it is respected during mapping, later changes as
well as resizes.

Furthermore, this commit fixes minor issues with how the hints are handled
during calculations.

fixes #2436

include/data.h

@@ -450,6 +450,10 @@ struct Window {
     int width_increment;
     int height_increment;
 
+    /* Minimum size specified for the window. */
+    int min_width;
+    int min_height;
+
     /* aspect ratio from WM_NORMAL_HINTS (MPlayer uses this for example) */
     double aspect_ratio;
 };