Add INSTALLS to lib.pro and process.pro

The gyp output libQt5WebEngineCore.so wasn't installed anywhere when
you did "make install". The QtWebEngineProcess is not built by gyp but
nonetheless was not installed anywhere.

Fixed by adding some INSTALLS to lib.pro and process.pro. I made the
lib.pro INSTALL unix-specific for now because I'm not sure how to write
"extra" INSTALLS for macx and win.

Change-Id: Ic127ab79c5788988b4d12f741eece7afc71cf6ce
Reviewed-by: Andras Becsi <andras.becsi@digia.com>