fix(UI): rename `qTox.desktop` → `qtox.desktop` to fix displaying icon

Apparently qTox's icon is not displayed under Plasma Wayland session
when its desktop file name differs from the executable name.

Fixes #4323.

cmake/Installation.cmake

@@ -39,7 +39,7 @@ if(APPLE)
 else()
   install(TARGETS ${PROJECT_NAME} RUNTIME DESTINATION "bin")
   install(FILES "res/qTox.appdata.xml" DESTINATION "share/appdata")
-  install(FILES "qTox.desktop" DESTINATION "share/applications")
+  install(FILES "qtox.desktop" DESTINATION "share/applications")
 
   # Install application icons according to the XDG spec
   set(ICON_SIZES 14 16 22 24 32 36 48 64 72 96 128 192 256 512)

qtox.pro

@@ -215,7 +215,7 @@ win32 {
         DATADIR = $$PREFIX/share
         target.path = $$BINDIR
         desktop.path = $$DATADIR/applications
-        desktop.files += qTox.desktop
+        desktop.files += qtox.desktop
         appdata.path = $$DATADIR/appdata
         appdata.files += res/qTox.appdata.xml
         INSTALLS += target desktop appdata
@@ -403,7 +403,7 @@ macx {
 freebsd {
     target.path  = $$PREFIX/bin
 
-    desktop.files = qTox.desktop
+    desktop.files = qtox.desktop
     desktop.path = $$PREFIX/share/applications
 
     icon.files = img/qtox.png

res/qTox.appdata.xml

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Copyleft 2016 SkyzohKey <skyzohkey@framasphere.org> -->
 <component type="desktop">
- <id>qTox.desktop</id>
+ <id>qtox.desktop</id>
  <metadata_license>CC-BY-SA-3.0</metadata_license>
  <project_license>GPL-3.0</project_license>
  <name>qTox</name>