Desktop: Add + to access 'Zoom In', allows to use the numpad (#2630)

* add + to access 'Zoom In', allows to use the numpad

This change aligns Joplin's behavior with all other apps that have shortcuts for `Zoom In`.
It therefore improves the user experience.

/ref

https://discourse.joplinapp.org/t/please-help-test-the-new-desktop-pre-release/6445/2?u=tessus
#2165 (comment)

* add comment

* improve comment

ElectronClient/app.js

@@ -1031,10 +1031,22 @@ class Application extends BaseApplication {
 					},
 					accelerator: 'CommandOrControl+0',
 				}, {
+					// There are 2 shortcuts for the action 'zoom in', mainly to increase the user experience.
+					// Most applications handle this the same way. These applications indicate Ctrl +, but actually mean Ctrl =.
+					// In fact they allow both: + and =. On the English keyboard layout - and = are used without the shift key.
+					// So to use Ctrl + would mean to use the shift key, but this is not the case in any of the apps that show Ctrl +.
+					// Additionally it allows the use of the plus key on the numpad.
 					label: _('Zoom In'),
 					click: () => {
 						Setting.incValue('windowContentZoomFactor', 10);
 					},
+					accelerator: 'CommandOrControl+Plus',
+				}, {
+					label: _('Zoom In'),
+					visible: false,
+					click: () => {
+						Setting.incValue('windowContentZoomFactor', 10);
+					},
 					accelerator: 'CommandOrControl+=',
 				}, {
 					label: _('Zoom Out'),