please add a11y descs + fix mnemonics clashes


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 Tested Window title : Add External Cluster
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Results of Accessibility test
 Doesn't implement Accessible :
   - none.
 No Accessible name :
   - none.
 No Accessible description :
   Class: javax.swing.JButton {  Browse... |  } 
   Class: javax.swing.JTextField {  Cluster directory: |  } 
   Class: org.netbeans.core.windows.services.NbDialog {  Add External Cluster |  } 
 Label with LABEL_FOR not set :
   - none.
 Components with no LABEL_FOR pointing to it :
   - none.
 Components with no mnemonic :
   - none.
 Components with wrong mnemonic (mnemonic isn't ASCII , label doesn't contain mnemonic):
   - none.
 Components with potential mnemonics conflict:
 - components with mnemonic 'Z' :
   Class: javax.swing.JButton {  Add ZIP/Folder... |  Add folder } 
   Class: javax.swing.JButton {  Add ZIP/Folder... |  Add folder } 
 - components with mnemonic 'U' :
   Class: javax.swing.JButton {  Move Up |  Move up button } 
   Class: javax.swing.JButton {  Move Up |  Move up button } 
 - components with mnemonic 'M' :
   Class: javax.swing.JButton {  Remove |  Remove platform } 
   Class: javax.swing.JButton {  Remove |  Remove platform } 
 - components with mnemonic 'D' :
   Class: javax.swing.JButton {  Move Down |  Move down button } 
   Class: javax.swing.JButton {  Move Down |  Move down button } 
 Components not reachable with tab traversal :
   - none.