Configure the select all behavior by OS instead of application template (#2260)

* Configure the select all behavior by OS instead of application template

* Partially revert 6e38465

Have the "select all" keybinding configurable by operating system as well as by application template

* Select all e-mails when no text window is currently focussed

app/keymaps/base-darwin.json

@@ -8,5 +8,7 @@
   "window:reload": "mod+alt+l",
   "window:toggle-dev-tools": "meta+alt+i",
   "window:attach-to-xcode": "meta+alt+k",
-  "window:open-mailsync-logs": "meta+shift+k"
+  "window:open-mailsync-logs": "meta+shift+k",
+
+  "multiselect-list:select-all": "command+a"
 }

app/keymaps/base-linux.json

@@ -3,5 +3,7 @@
   "core:paste": "shift+insert",
   "window:toggle-full-screen": "f11",
   "window:reload": "mod+alt+l",
-  "window:toggle-dev-tools": "mod+alt+i"
+  "window:toggle-dev-tools": "mod+alt+i",
+  
+  "multiselect-list:select-all": "ctrl+a"
 }

app/keymaps/base-win32.json

@@ -1,5 +1,7 @@
 {
   "window:toggle-full-screen": "f11",
   "window:reload": "ctrl+shift+r",
-  "window:toggle-dev-tools": "ctrl+shift+i"
+  "window:toggle-dev-tools": "ctrl+shift+i",
+  
+  "multiselect-list:select-all": "ctrl+a"
 }

app/src/window-event-handler.ts

@@ -146,7 +146,7 @@ export default class WindowEventHandler {
       'core:undo': e => (isTextInput(e.target) ? webContents.undo() : getUndoStore().undo()),
       'core:redo': e => (isTextInput(e.target) ? webContents.redo() : getUndoStore().redo()),
       'core:select-all': e =>
-        isIFrame(e.target) || isTextInput(e.target) ? webContents.selectAll() : null,
+        isIFrame(e.target) || isTextInput(e.target) ? webContents.selectAll() : AppEnv.commands.dispatch('multiselect-list:select-all'),
     });
 
     // "Pinch to zoom" on the Mac gets translated by the system into a