remove restrictions on mapmodel rotation

Until now, the angles of mapmodels (and ctf-flags) were rounded to the
nearest 15-degree-step during rendering. This patch moves the rounding
to the time of placement of the model. Now the angle can be changed freely
after the model has been placed.

This commit also changes the angle resolution to 0.25 degree.

When changing the angle (press "1" + scrollwheel), additionally pressing
editmeta (a.k.a. LCTRL) changes the increments of the wheel from 1 to 10.