Replace tabs with spaces
Change the plugin's description line length

Slightly reworked the plugin's description line breaks, as this length
still easily fits into the window's constrains, but at the same moment is
less lines, thus making the dialog usable also on monitors with lower
vertical resolution (tested with Inkscape 0.48.4 r9939).
Auto-height functionality

The auto height option sets the extruded height for a specific polygon
based on its ID. If the polygon's ID is in either name_N_mm or name_N_N_mm
format, the NNN_(NNN_)mm part of the name is translated into height in
millimeters.

Otherwise, the height parameter is used.

Examples:
rect543_1_mm -> 1 mm height
poly456_12_5_mm -> 12.5 mm height
circle2345_3 -> according to the height parameter
Merge branch 'rajcze-master'