locking: aquire & release the yum lock for each backend operation

the old behaviour was to keep the lock during the whole runtime of the
backend. this made it impossible to run yumex and at the same time install
something with yum via command line
get_packages: add a parameter to disable backend package cache (used to search for updates)
progress dialog: ignore the close button
add 'close to hide window' option & improve closing behaviour of progress window
change default config: don't ask to exit yumex after a transaction
Merge pull request #7 from bkueng/update_checking

Update checking
fix 'yumex install' make yumex crash, when trying to start the root backend
make sure transaction confirmation is shown when running yumex -X install some-name.rpm
pack the yum messages when sending from backend to frontend, so we don't get utf8 conversion error on non english locale messages (rhbz # 882527 and others)
Make sure that confirmation dialog i always shown, even if main window is non visible
don't get full package list, when starting backend in root mode (save time)
catch and ignore all exception, when checking network state from networkmanager (rhbz #954998)
Fix missing # in comment
fix:  local variable 'alist' referenced before assignment, when all packages is skipped in a transaction (RHBZ #906237)
make the backend-launcher more error proff
catch expections in pack/unpack
Added InstallAll & RemoveAll Button to install/remove all packages in a given group
IOError, dont have an err attribute, so don't try to get it (rhbz #823315)
Dont show a error dialog, if backend is not running, when we close the application (rhbz #996901))