Don't draw textfields below a certain width

## Abstract
Normally, the wmhints limited the window to a minwidth and minheight. However, according to the ICCCM-conventions, it's not good practice relying on the wmhints to mask undefined behaviour.

## How to reproduce
The bug currently exists on all platforms and is reproducable by removing the size-hints on the window (comment out xlib/main.c:696 for XLib-testing).
Now, scale the window down arbitrarily and you'll sooner or later notice that the window is frozen and won't redraw.

## Explanation
Somehow, there's an issue drawing PANEL_EDITs (textfields) smaller than a certain width (somehow the width of the scrollbar).
This patch prevents the textfield from being drawn when it reaches a certain margin.

Given this margin is never reached in floating state, and only used to fix freezes in tiling-wm's and other weird circumstances you can read in the other issue-reports, this won't happen in most cases.
However, it sure will fix freezes that have already been reported numerous times.