Add tooltips to hotkey dialog buttons

THis is done in the generic widget, so other users can
benefit from it.

common/dialogs/panel_hotkeys_editor.cpp

@@ -101,6 +101,7 @@ void PANEL_HOTKEYS_EDITOR::installButtons( wxSizer* aSizer )
         {
             wxID_RESET,
             _( "Reset Hotkeys" ),
+            _( "Undo all changes made so far in this dialog" ),
             [this]( wxCommandEvent ){
                 m_hotkeyListCtrl->ResetAllHotkeys( false );
             }
@@ -108,6 +109,7 @@ void PANEL_HOTKEYS_EDITOR::installButtons( wxSizer* aSizer )
         {
             wxID_ANY,
             _( "Set to Defaults" ),
+            _( "Set all hotkeys to the built-in KiCad defaults" ),
             [this]( wxCommandEvent ){
                 m_hotkeyListCtrl->ResetAllHotkeys( true );
             }
@@ -118,6 +120,7 @@ void PANEL_HOTKEYS_EDITOR::installButtons( wxSizer* aSizer )
         {
             wxID_ANY,
             _( "Import..." ),
+            _( "Import hotkey definitions from an external file, replacing the current values" ),
             [this]( wxCommandEvent ){
                 m_frame->ImportHotkeyConfigFromFile( m_hotkeys, m_nickname );
             }
@@ -125,6 +128,7 @@ void PANEL_HOTKEYS_EDITOR::installButtons( wxSizer* aSizer )
         {
             wxID_ANY,
             _( "Export..." ),
+            _( "Export these hotkey definitions to an external file" ),
             [this]( wxCommandEvent ){
                 m_frame->ExportHotkeyConfigToFile( m_hotkeys, m_nickname );
             }

common/widgets/button_row_panel.cpp

@@ -68,6 +68,8 @@ void BUTTON_ROW_PANEL::addButtons( bool aLeft, const BTN_DEF_LIST& aDefs )
         if( ( aLeft ) || ( !aLeft && i < aDefs.size() - 1 ) )
             this_style |= wxRIGHT;
 
+        btn->SetToolTip( def.m_tooltip );
+
         m_sizer->Add( btn, btn_proportion, this_style, btn_margin );
 
         btn->Bind( wxEVT_COMMAND_BUTTON_CLICKED, def.m_callback );

include/widgets/button_row_panel.h

@@ -63,6 +63,11 @@ public:
          */
         wxString        m_text;
 
+        /**
+         * Button tooltip text - empty string for no tooltip
+         */
+        wxString        m_tooltip;
+
         /**
          * The callback fired when the button is clicked. Can be nullptr,
          * but then the button is useless.