JBIDE-14735 Improve text formatting when dropping widgets from Palette view

Issue is fixed. Prev. sibling/parent tag is used to calculate the indent of the template text
that is to be inserted. 'Use spaces' option is taken into account. All indents that are inserted are
recalculated by using tabs (until it's possible) then spaces.
JUnit Test is added for the issue: org.jboss.tools.common.model.ui.views.palette.test.PaletteInsertHelperTest
Typoo in constant names are fixed