File numbering stops at 999

[Edgar Maloverian]

Shutter 0.94, Rev.1290, on kubuntu 18 LTS

File naming goes as following:
```-rw-rw-r-- 1 ets ets 58096 Mar 31 12:15 Selection_992.jpg
-rw-rw-r-- 1 ets ets 226835 Mar 31 13:33 Selection_993.jpg
-rw-rw-r-- 1 ets ets 10899 Mar 31 13:58 Selection_994.jpg
-rw-rw-r-- 1 ets ets 140422 Mar 31 14:50 Selection_995.jpg
-rw-rw-r-- 1 ets ets 7308 Mar 31 14:52 Selection_996.jpg
-rw-rw-r-- 1 ets ets 24873 Mar 31 14:54 Selection_997.jpg
-rw-rw-r-- 1 ets ets 102549 Mar 31 21:33 Selection_998.jpg
-rw-rw-r-- 1 ets ets 163083 Apr 1 02:20 'Selection_999(001).jpg'
-rw-rw-r-- 1 ets ets 185898 Apr 2 12:07 'Selection_999(002).jpg'
-rw-rw-r-- 1 ets ets 103269 Apr 2 16:04 'Selection_999(003).jpg'
-rw-rw-r-- 1 ets ets 26165 Apr 2 16:22 'Selection_999(004).jpg'
-rw-rw-r-- 1 ets ets 111454 Apr 2 17:57 'Selection_999(005).jpg'
-rw-rw-r-- 1 ets ets 92572 Apr 2 22:36 'Selection_999(006).jpg'
-rw-rw-r-- 1 ets ets 19639 Apr 3 11:14 'Selection_999(007).jpg'
-rw-rw-r-- 1 ets ets 107236 Apr 3 11:14 'Selection_999(008).jpg'
-rw-rw-r-- 1 ets ets 26769 Apr 3 13:10 'Selection_999(009).jpg'
```
Expected default behaviour would be "Selection_1000.jpg"