use the standard system download path for file downloads

QFileDialog's dir arg (which was set to the incoming file name from the
Matrix download) can take a full path to suggest. By prepending
QStandardPaths::DownloadLocation, it opens to the system's download
folder and proposes the filename as the download name.

Using QStandardPaths should make this work on other platforms, and from
what I read, its possible for this to return an empty string on
platforms where it doesn't support it, so this should essentially
revert to the previous functionality if Qt can't determine the system's
download location.

Add Japanese translation

markdown -> Markdown

Fix image overlay appearing on the wrong screen.

thanks adasauce!

it was causing the full screen image overlay to appear on the "primary
display" vs. the actual display nheko is running on.

removing the move() call makes the overlay follow the window.

The ISO 639-1 code of Japanese is not "jp" but "ja".

Update the logo to not be so asymmetric and add a scalable svg. I hope I fixed all the scaling issues, where the logo isn't pixel aligned in most resolutions. Also fixes an issue, where the blue bar seemed to be misaligned in some resolutions?