QtIFW

Allow specifying the installer size in 'em' or 'ex' units

The WizardDefaultWidth and WizardDefaultHeight configuration settings
always expected pixel values. This made the installer look somewhat
awkward on high DPI displays, in which case the ratio between the font
size and the installer window size was such that the fonts looked very
big.

Let's fix this by allowing to specify the width/height of the installer
using units which are defined in terms of the font size, namely 'em'
("The width of the letter M") and 'ex' ("The width of the letter x").
'px' is supported as well and means the same thing as not specifying any
unit at all: the given size is defined in pixels.

We choose to *not* use the font width for 'em' and 'ex' to be consistent
with what the Qt CSS parser does (see src/gui/text/qcssparser.cpp),
which adheres to what the W3C document at
https://www.w3.org/WAI/GL/css2em.htm describes.

Change-Id: Iaeb5a29c79d437ef4b956cb318158181f6289ec9
Reviewed-by: Karsten Heimrich <karsten.heimrich@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>