Fix three uninitialised-value bugs found by Valgrind

1. machine_info.h: zero-initialise Screen struct members
   Max_width, Max_height, count, width[] and height[] were bare
   int32_t with no initialiser. The comparisons in
   init_get_Screen_info() read them before any write, producing
   undefined behaviour flagged by Valgrind as 'Conditional jump
   or move depends on uninitialised value(s)'.

2. main.cpp: pre-initialise MachineInfo on the main thread
   MachineInfo::instance() was first called inside QtConcurrent::run(),
   causing its constructor (which calls qApp->screens()) to run on a
   background thread. QScreen methods are not thread-safe in Qt5.
   Calling instance() once on the main thread before the worker
   launches guarantees the singleton is fully built first; subsequent
   calls from the worker just return the cached pointer.

3. qetdiagrameditor.h: move m_first_show before the QActionGroup members
   C++ initialises members in declaration order. m_first_show was
   declared after the QActionGroup members (line 256 vs 168). During
   construction of m_row_column_actions_group(this), Qt dispatches a
   QObject parent-change event that reaches QETDiagramEditor::event(),
   which reads m_first_show before it has been initialised.
   Moving the declaration to the top of the first private: block
   ensures it is initialised before any member that can trigger events.

All three found via Valgrind --tool=memcheck on Ubuntu 22.04 / Qt 5.15.3.
Relates-to: PR #514 (same QtConcurrent thread-safety pattern).

sources/machine_info.h

@@ -75,11 +75,11 @@ class MachineInfo
 	{
 		struct Screen
 		{
-			int32_t count;
-			int32_t width[10];
-			int32_t height[10];
-			int32_t Max_width;
-			int32_t Max_height;
+			int32_t count     = 0;
+			int32_t width[10] = {};
+			int32_t height[10]= {};
+			int32_t Max_width = 0;
+			int32_t Max_height= 0;
 		}screen;
 		struct Built
 		{