Add function to remove specific custom buttons by label

[schilgod]

Creating new PR as old PR #4219 was closed.
@rmehta Hope that you will accept this.

Some reasons this PR is helpful,

1. This will make the code in custom apps cleaner and any changes on Frappe wont affect custom apps in future, if any.
2. This is useful when we need to remove one or more buttons, but not all.
   For example, in Material Request, currently we don't need the buttons under "Make", but we do need Stop button. We also have a custom button for creating Purchase Order(s) for items w.r.t Supplier, not a single Supplier as is case now.
3. Allows to show/hide buttons based on permissions.

Function to remove custom button by label

Example :

1. In Material Request, to remove "Stop" button, can be achieved by calling function as below,
   frappe.ui.form.on("Material Request", {
   onload: function(frm) {
   var buttons = [ [ "Stop", "" ] ];
   frm.remove_custom_button(buttons);

    //OR
    //var buttons = [ [ "Stop" ] ]; (since it's not inside group of buttons.)
    //frm.remove_custom_button(buttons);
   

   }
   });

2. In Material Request, to remove "Purchase Order" button, can be achieved by calling function as below,
   frappe.ui.form.on("Material Request", {
   onload: function(frm) {
   var buttons = [ [ "Purchase Order", "Make" ] ];
   frm.remove_custom_button(buttons);
   }
   });

3. Remove multiple buttons,
   frappe.ui.form.on("Material Request", {
   onload: function(frm) {
   var buttons = [ [ "Purchase Order", "Make" ], [ "Supplier Quotation", "Make" ] ];
   frm.remove_custom_button(buttons);
   }
   });

[netchampfaris]

@schilgod

To remove a simple custom button, I have to use
frm.remove_custom_button([['Close']]).

Can we make it more API friendly?

For example, these should work:

frm.remove_custom_button('Close');

frm.remove_custom_button('Purchase Order', 'Make');

frm.remove_custom_button(['Purchase Order', 'Sales Order'], 'Make');

[schilgod]

thanks @netchampfaris