murmur: build fixes for win32-static.
Fix path in skin selection dialog

* Open skin selection dialog with sane path
** if empty
*** installation dir on windows
*** data dir on linux
*** in skins subfolder, if it exists
** Use currently set path if set
Open soundfile selection dialog with current path
mumble, murmur: final adjustments for the win32-static buildenv.

With this change, CONFIG(static) on Windows will cause the Mumble client's
application logic to be built into a .DLL called mumble_app.dll
(based on pcgod's previous DLL changeset).

Since src/mumble will now be built as a DLL, a wrapper executable is available
in src/mumble_exe. This wrapper is currently implemented such that it will
load mumble_app.dll from the directory that it resides in.

This means that when building statically, src/mumble and src/mumble_exe will
now give us the following products:

  src/mumble:      mumble_app.dll
  src/mumble_exe:  mumble.exe

Along with the two major points above, this change also adds a Python script
to the build, 'gen-mumble_app-qt-def.py', whose job is to construct a module
definition (.def) file for mumble_app.dll. The generated module definition
lists the Qt symbols that are needed for the manual positioning plugin to work.

If we need to expose more symbols in the future (say we want to implement
more plugin kinds than the current positional audio plugins), we now have
the infrastructure in place to do that.
plugins/manual: only link against mumble_app when doing a static win32 build.
plugins/manual: use qt_dynamic_lookup instead of static to guard linking against mumble_app.dll.
winpaths_static.pri: use version-less protobuf path.
compiler.pri: allow version-less Boost to be used on Windows.

This allows Mumble to build with Boost that has been
configured with the --layout=tagged or --layout=system
options.

The win32-static build environment uses this kind of
Boost install.

This means we'll be able to keep Boost up-to-date without
modifying compiler.pri each time.
installer: automatically build a static installer when in the win32-static build env.
Merge branch 'master' of https://github.com/mumble-voip/mumble