Use percent encoding when creating urls for UDS entries

Summary:
Otherwise it won't be possible to open gdrive files that contain special
characters (such as `#`) in their path.

This is not handled by libkgapi, which just copies whatever is in the
JSON from Google, and Google does not seem to encode the filenames
either. So we encode the filenames at the kio-gdrive level, where we
actually play with QUrls.

We do so by actually creating a QUrl (which takes care of
percent-encoding in `setPath()`) and then we convert it to a string.

While at it, use faster concatenation rather than arg parsing.

BUG: 395590
FIXED-IN: 1.2.4

Test Plan:
Try to open a file with `#` in the name.
Check whether files with normal names still work.

Reviewers: dvratil, dfaure

Differential Revision: https://phabricator.kde.org/D13691