Various cleanups.

- Update input handling to retain SDL_TEXTINPUT after Tab insertion for better platform consistency.
- Allow multiple app instances in macOS by modifying `Info.plist`.
- Bump version to 1.3.8-alpha.

cmake/Info.plist.in

@@ -24,5 +24,8 @@
     <string>10.13</string>
     <key>NSHighResolutionCapable</key>
     <true/>
+    <!-- Allow running multiple instances of the app -->
+    <key>LSMultipleInstancesProhibited</key>
+    <false/>
 </dict>
 </plist>