Fix Filter Panel display when going to/from empty panel

The JPanel's default FlowLayout adds 5px paddings on top and bottom.
Without any filter buttons it is of no concern as the panel with(out)
buttons has a desired height of 0, so it is stretched out to the size of
the container. When the filter button appears, these paddings are
applied and this caused the FilterPanel to become 10px taller.

The fix is to remove the default paddings from the inner JPanel.
Instead, some paddings were added to the FilterPanel itself to achieve
a more pleasant look.

Additionally, a "revalidate()+repaint()" spell is now used when the
filter is added. Without this, the change in height is not propagated
to the FilterPanel (thought there is no more change in height).

Issue: #218

src/name/mlopatkin/andlogview/ui/filterpanel/FilterPanel.java

@@ -118,7 +118,8 @@ public void onFilterAdded(PanelFilterView newFilter) {
         buttonByFilter.put(newFilter, button);
         content.add(button);
         menuHandler.addPopup(button);
-        validate();
+        revalidate();
+        repaint();
     }
 
     @Override

src/name/mlopatkin/andlogview/ui/filterpanel/FilterPanelUi.java

@@ -30,6 +30,7 @@
 import javax.swing.JViewport;
 import javax.swing.SwingConstants;
 import javax.swing.UIManager;
+import javax.swing.border.EmptyBorder;
 
 /**
  * FilterPanel implementation. Generated class from WindowBuilder editor.
@@ -48,6 +49,7 @@ class FilterPanelUi extends JPanel {
     protected final JViewport contentViewport;
 
     FilterPanelUi() {
+        setBorder(new EmptyBorder(5, 5, 5, 5));
         setLayout(new BoxLayout(this, BoxLayout.LINE_AXIS));
         setBackground(UIManager.getColor("ToolBar.background"));
 
@@ -68,9 +70,9 @@ class FilterPanelUi extends JPanel {
         add(Box.createRigidArea(new Dimension(SEPARATOR_WIDTH, SEPARATOR_HEIGHT)));
 
         content = new JPanel();
+        content.setLayout(new FlowLayout(FlowLayout.LEFT, 5, 0));
         content.setBackground(UIManager.getColor("ToolBar.background"));
         content.setBorder(UiHelper.NO_BORDER);
-        ((FlowLayout) content.getLayout()).setAlignment(FlowLayout.LEFT);
         JScrollPane scrollPane =
                 new JScrollPane(content, JScrollPane.VERTICAL_SCROLLBAR_NEVER, JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
         scrollPane.setBorder(UiHelper.NO_BORDER);