open the current path of the cache folder now
[NFC] delete filenetworktask safer

and eliminate some annoying warnings
QCoreApplication::postEvent: Unexpected null receiver

reasoning: QScopePointer use delete operator to delete the resource directly
when it is destoryed. this will force it to use deleteLater method instead.
add create directory support, backend part
add create directory support, ui part
insert the created directory to the front instead of the back
provide better information when error occurs
Merge pull request #330 from haiwen/patch-27-open-current-cache-folder

open the current path of the cache folder now
Merge pull request #332 from haiwen/delete_scopepointer_safer

[NFC] delete filenetworktask safer
pdf files are also opened by Preview app on mac
improve some lines' code quality
fix the last modifyed time of uploaded file
Merge pull request #334 from haiwen/fix_mtime_of_uploaded_file

fix the last modifyed time of uploaded file
prompt a confirmation dialog if the user is going to overwrite file
Merge pull request #335 from haiwen/prompt_a_confirmation_dialog_if_overwriting

prompt a confirmation dialog if the user is going to overwrite file
fix showInGraphicShell on windows

QProcess::startDetached wraps arguments that contain spaces in quotes on
windows.
Merge pull request #336 from haiwen/fix_showingraphicshell_on_windows

fix showInGraphicShell on windows
replace NO_ERROR that is defined as a macro on windows
remove libzdb and other unused dependencies which are unnessary from ci script
don't include headers in anonyous namespaces

it is evil to do this and what is a plus is that it breaks cross-compiling on linux
Merge pull request #337 from haiwen/fix_mingw_build

replace NO_ERROR that is defined as a macro on windows