Increase maximum size of system icons

System icons were limited to a size 24x24 pixels, while the embedded
icons are 32x32 pixels big.
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 #2027 from pmzqla/system_icons

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

Optimize includes
Unify code indentation
Add texts to the icons on the toolbar

The toolbar follows the system style to display texts associated to
icons. Since no specific text is provided, the text used for the action
associated to the button will be used. This could make some of the
buttons rather big.

Add short texts to the icons so that each button uses a reasonable
amount of space.
Remove unneeded tooltip

A tooltip will be generated using the text of the action anyway.
Add context menu to allow minor toolbar customizations

Add a simple context menu to the toolbar to set the size of the icons
and the position of text.