Increase maximum size of system icons

System icons were limited to a size 24x24 pixels, while the embedded
icons are 32x32 pixels big.
Make Windows icons suitable for high dpi screens

The maximum size of the icons is now 256x256 pixels so that
Windows can correctly scale them.

The 256x256 layer for the main icon was created upscaling the
192x192 png available in the source tree. Hence, the icon might
not be perfect at the maximum size.

The icon for the file association was made from scratch and it's
visually similar to the previous icon.
Set minimum width of the left panel in the preferences

Change also the default width and set it to the minimum.
This minimum width prevents the horizontal scrollbar from appearing.

The size of the items in the list depends on the Qt style, so the
left panel could be few pixels larger than required with some of them.
Don't stretch the last section in the transfer list

Since the content of some sections is right aligned, automatically
resizing the width of the last one to fill the header could be
sometimes undesired.

Let the user choose the width of each section and never change his
preference.
Remove unneccessary includes of libtorrent in misc.h
Remove inclusion of <QThread> from misc.h
Make alertdispatcher.h self contained and replace include libtorrent/session with forward declaration
Remove inclusion of alertdispatch.h from qbtsession.h
Replace include of <libtorrent/session.hpp> with forward declaration
Remove inclusion of "qtracker.h" in "qbtsession.h"
Speedup compilation speed

libtorrent has a relatively heavy headers, that take lots of time to
process. This commit removes unnecessary includes of libtorrent headers
and replaces them with forward declarations.

I had to move some functions in QBtSession from slots to regular
functions because moc'ed file want to see complete types of all
parameters of slots.

"time make" of full rebuild before this series of commits:

real    13m35.937s
user    12m1.295s
sys     1m25.908s

after:

real    10m54.390s
user    9m31.167s
sys     1m12.580s
Merge pull request #2034 from pmzqla/sections

Don't stretch the last section in the transfer list
Merge pull request #2031 from pmzqla/panel

Set minimum width of the left panel in the preferences
Allow relative torrent paths when qBittorrent is already running

Adding new torrents from the command line while qBittorrent was
already running was possible only providing the absolute path to
the file.
Merge pull request #2035 from pmzqla/torrentpath

Allow relative torrent paths when qBittorrent is already running
Merge pull request #2026 from pmzqla/master

Make Windows icons suitable for high dpi screens
Merge pull request #2027 from pmzqla/system_icons

Increase maximum size of system icons
Merge pull request #2042 from sorokin/optimize-includes

Optimize includes
Use the correct character encoding for exceptions coming from libtorrent.
Optimize misc::naturalSort()

Previous implementation used QRegExp to find a first digit. That is
utterly ineffective.

When torrent list is sorted by name (a column that uses the naturalSort() for
comparison), naturalSort could take 18.6% of the time of the UI thread. Optimize it
so now it takes 1% of the time of the UI thread.