misc clean up and arch PKGBUID

1. add arch PKGBUILD for abs building, need upload to AUR
2. clean up directories for packaging, remove useless files.
3. change version to 2.0.20
clean up useless files
fix Qt5 porting issue, not complete

1. replace Q_WS_* with Q_OS_*, because Qt5 remove these macros already.
2. add Q_OS_* macros to moc when compile, qmake will add for themself,
but not cmake.
Qt5 porting issue fix for mac, not complete

1. Qt5 can't find QT_MAC_USE_COCOA
2. Qt5 remove QMacCocoaContainer from mainstream, but still leave
header on include directory, should use external project "qtmacextras",
it's confused me a lot!!!
3. Qt5 changed moc include name strategy, mix cpp and objc method is
different, separate headers should be used.
fix qt5 porting issue for mac

1. fix bundle building issue
2. disable mac native search field cause qt5 can't support it currently.
3. use qt5 native scrollbar, it's more like mac native scrollbar.

Still need working:
1. scroll on document list speed is poor on mac! use pixmap cache for
drawing instead!
2. mac fullscreen is supported on Qt5, use native solution.
3. mac native toolbar is not supported by Qt5 currently(Qt5 v5.1.1).
build on linux
fix build issue on mac

1. install qt internal translation.
2. macdeployqt resolve.
fix about dialog logo issue
fix qt5 porting issue for mac

use QPixmapCache for buffered drawing of document list view cause
scroll or touchpad action on OSX is so slow.

still need more work on group document list drawing.
fix qt5 background drawing issue of info window

still need more work on it.
not clear password when logout
bug fix: folder rename issue

user rename folders not record on database with AddExtraFolder.
bug fix: document list drawing issue

clear QPixmapCache while document list size change to request
drawing buffer refresh.
bug fix: queued download object data issue

Qt5 remove terminated signal from QThread, use "finished"
signal instead.
bug fix: skip useless messages issue

document data may deleted from database while messages still
stand alone, skip these messages as response, we should delete
them in the future.
reopen FTS rebuild menu item selection.

at some extreme cases, clucene index will be broken and affect
main app crash at start up!

reopen this choose for user who meet this issue.

we still need separate clucene index work to another process
to avoid main thread crash!!!
bug fix: xml-rpc may return nil on dt_deleted

reported by a user from US who come across this issue.
normal user's dt_deleted field seems always valid as normal.
update README
update README
update README