Fix webview doesn't get focus correctly

For #231

webview.focus() was used to focus on the webview, but it's DOM element function.
So only the tag was focused in the host window, and its webContents was not focused.
As the result, the shortcuts of Mattermost server didn't work in some cases.
Electron is upgraded to 1.2.8 because of webContents.isFocused().

CHANGELOG.md

@@ -32,6 +32,7 @@ Release date: TBD
 
 ### Bug Fixes
 - Fixed an issue where the maximized state of the app window was lost in some cases.
+- Fixed an issue some shortcuts didn't work when switching applications or tabs in some cases.
 
 #### Windows
 - Fixed pixelated application icons in top left of the window.