Fix IMF manager issues.

The text underline while pre-edit text differs from other solutions/toolkits i.e EFL or Android.

One use case that looks different is:
1) Type a word
2) Tap somewhere inside the word
3) Add more text. At this point the underline is visible in Dali but not in EFL or Android.

In addition if the text added in the step 3) is deleted, the underline remains in one character, which is wrong.

Another use case:
1) Type a word
2) Tap at the end of the last word of the text
3) Add more text. At this point the whole last word should be underlined as pre-edit. Sometimes it happens, sometimes it doesn't.

Change-Id: Ib0c76157260776296dfec34680bd200c1b3268f1
Signed-off-by: Victor Cebollada <v.cebollada@samsung.com>