Fixing WiX UI extension screens: texts, icons, margins, spacing between elements.
Summary of changes:
 - Added a label (in bold) before input fields.
 - Fixed margins and space between elements. There were inconsitencies.
 - Fixed naming inconsistencies in code - e.g., strings in the .wxl for "ServiceCredDlg" started with "ServiceDlg_".
 - Merged into a unique text the 2 "description" labels in DbCreateCredDlg and RuntimeDbCredDlg.
 - Cleaned up unused strings, like texts for 'Browse' buttons that don't exist.
 - Created new error/success icons.