Spreadsheet: Change paste action's enabled state on clipboard change

Previously, the paste action was always enabled and always assumed that
anything was selected, which led to a crash by clicking the paste action
right after the application startup.

This patch will automatically enable/disable the paste action depending
on whether a selection exists (it usually does, except on the app launch
and after adding a new tab) and if the clipboard mime type is a text/
group.

So no, you can't paste an image into the app anymore, even though this
mostly froze the app before...

Userland/Applications/Spreadsheet/SpreadsheetWidget.h

@@ -9,12 +9,15 @@
 #include "SpreadsheetView.h"
 #include "Workbook.h"
 #include <AK/NonnullRefPtrVector.h>
+#include <LibGUI/Clipboard.h>
 #include <LibGUI/TabWidget.h>
 #include <LibGUI/Widget.h>
 
 namespace Spreadsheet {
 
-class SpreadsheetWidget final : public GUI::Widget {
+class SpreadsheetWidget final
+    : public GUI::Widget
+    , public GUI::Clipboard::ClipboardClient {
     C_OBJECT(SpreadsheetWidget);
 
 public:
@@ -49,8 +52,12 @@ public:
     auto& undo_stack() { return m_undo_stack; }
 
 private:
+    // ^GUI::Widget
     virtual void resize_event(GUI::ResizeEvent&) override;
 
+    // ^GUI::Clipboard::ClipboardClient
+    virtual void clipboard_content_did_change(String const& mime_type) override;
+
     explicit SpreadsheetWidget(GUI::Window& window, NonnullRefPtrVector<Sheet>&& sheets = {}, bool should_add_sheet_if_empty = true);
 
     void setup_tabs(NonnullRefPtrVector<Sheet> new_sheets);